A digital broadcast transmitter performs AGC (Automatic Gain Control) which controls an output level at a fixed level by extracting and monitoring part of transmission outputs, performing automatic adjustment which carries out feedback controlling of a gain of the amplifier such that this monitoring signal meets a predetermined AGC reference value, such that a transmission output level settles accurately in an predetermined level even when an amplifier of the transmitter is influenced by a temperature or temporal changes.
FIG. 7 is a functional block diagram for explaining a configuration of the digital broadcast transmitting device. The digital broadcast transmitting device which transmits broadcast radio waves has an exciter ex (EX) which receives an input of a TS (Transport Stream) signal from a net signal and the like from a studio device, a server and an affiliated station and converts the TS signal into a radio frequency signal, and a power amplifier PA which amplifies the converted radio frequency signal to a predetermined signal level and outputs the radio frequency signal from an antenna.
For digital broadcast, the power amplifier PA having high linearity and a wide dynamic range is used to transmit a quadrature modulation wave, i.e., an OFDM modulation wave, and the AGC function performs important operation compensation. However, the AGC function is performed by analog circuit processing of using a differential amplifier to compare a direct current signal obtained by detecting and rectifying a feedback signal as an output level signal and the AGC reference value which is a direct current voltage, and using a variable attenuator or the like to adjust an output of a difference signal as an error signal.
In some cases, test operations of the digital broadcast transmitting device are performed at a lower test output than a steady operation output upon a maintenance operation or the like. In this case, the above direct current AGC reference value is fixed. Therefore, when test operations are carried out in this state, the AGC function performs an operation of increasing a gain of an amplifier or decreasing an attenuation amount to increase an output, and therefore it is necessary to adjust the reference value.
FIG. 8 is a functional block diagram of the exciter ex of the digital broadcast transmitting device in which the AGC function is provided to the conventional exciter and which controls an output of the power amplifier PA at a fixed level.
In FIG. 8, the exciter ex has a digital processing unit dp which converts a broadcast TS signal to be input, into a radio frequency signal, and an analog processing unit ap which converts the converted radio frequency signal into a modulation signal of a broadcast radio wave format, applies AGC to the modulation signal to adjust the modulation signal at a predetermined signal level and outputs the modulation signal.
FIG. 9 is a functional block diagram of an AGCAmp 52 included in the analog processing unit ap. The analog processing unit ap outputs to the power amplifier PA a signal which is set to a predetermined level by an output monitoring unit 50 and which passes through a VATT (variable attenuator) 60. Further, a DET 5 receives an input of a feedback signal of a transmission radio wave from the power amplifier PA, detects and rectifies the feedback signal, converts the feedback signal into a direct current detection level signal and outputs the detection level signal to the AGCAmp 52. The AGCAmp 52 has a reference voltage source r, compares a reference voltage and the detection level signal, and outputs a VATT 60 control signal to minimize a difference between the reference voltage and the detection level signal.
A test operation is carried out by, for example, increasing the attenuation amount of the VATT 60 by 3 dB and decreasing a transmission output, and reducing the attenuation amount of a VATT 61 by 3 dB to match the detection level signal with the reference voltage. By so doing, it is possible to perform a required AGC operation even upon test operations. A great number of devices including relay devices are installed for a digital broadcast transmitting device. However, the digital broadcast transmitting device needs to save a space, save power, and easily adjust and set an output at a limited transmission space of a remote station.
However, the VATTs 60 and 61 which are analog circuit functional parts which carry out the AGC function, and therefore a device scale becomes large. In addition, an amplifying unit 4 suppresses an output at a fixed level, and therefore consumes unnecessary power. Further, when a plurality of levels needs to be set for test operations, adjusting and setting the VATTs 60 and 61 are bothersome. Meanwhile, in some cases, an output of a digital broadcast transmitting device is set to enable adjustment of coverage according to conditions of a place at which a transmitter is installed. Therefore, there is a problem that it is necessary to provide a reference voltage according to respective outputs or provide multiple types of default values of the VATT 61.